A polyester resin is used in many utilities due to its functional usefulness. For example, the polyester resin is used in fibers for clothing, materials, and medical services, films of wrapping materials, electrically insulating materials, and magnetic recording materials, and resin products. Among polyester resins, polyethylene terephthalate is excellent in general use, and practicability, and is suitably used.
In general, polyethylene terephthalate is produced from terephthalic acid or its ester-forming derivative, and ethylene glycol. In an industrial process for producing a polymer of a high molecular weight, an antimony compound is used as a polycondensation catalyst is widely used.
However, polyethylene terephthalate produced by using an antimony catalyst has some not preferable properties as described below.
For example, it is known that, when a polymer obtained by using an antimony catalyst is melt-spun into a fiber, the residue is sedimented around a spinneret pore. When this sedimentation progresses, since this becomes a cause for generation of a defect in a filament, necessity of removal at an appropriate time is generated. In addition, such the polyethylene terephthalate has not preferable properties that the antimony catalyst residue in a polymer easily becomes relatively greatly particulate, this becomes an extraneous material, and this becomes a cause for increase in a filtration pressure of a filter at molding processing, and yarn breakage upon spinning, or film breakage at film making, resulting in one cause for reduction in operability.
In addition, in film utility, an extremely flat film surface shape is desired mainly in a film for a magnetic recording medium in recent years. The extraneous material due to the antimony catalyst residue forms a coarse protrusion on such the film surface, being not preferable.
Under the aforementioned circumstances, a method for producing polyethylene terephthalate without using an antimony catalyst is desired. As a polycondensation catalyst other than an antimony compound, a germanium compound is known, but since the germanium compound is small in reserves, and rare, it is difficult to use the germanium compound for general-use.
In response to this problem, use of a titanium compound as a polymerization catalyst is being studied vigorously. Since the titanium compound has the higher catalyst activity, and the desired catalyst activity can be obtained by addition of a smaller amount as compared with the antimony compound, generation of an extraneous particle, and spinneret contamination can be suppressed. However, when the titanium compound is used as a polymerization catalyst, a side reaction is promoted due to the high activity and, as a result, a problem arises that thermal stability of the resulting polymer is deteriorated, and the polymer is colored yellow. Coloring of a polymer with yellowness is not the preferable property, for example, when the polyester is used in a fiber, particularly a fiber for clothing. Yellowness becomes problematic also when the polyester is used in a film, particularly an optical film.
In response to such the problem, improvement in heat resistance and a color tone of the polymer by addition of the titanium compound with a phosphorus compound is being studied widely. This method is to suppress the too high activity of titanium with the phosphorus compound to improve heat resistance and a color tone of the polymer. For example, Japanese Patent Application Laid Open (JP-A) No. 2004-292657 discloses a method of adding a phosphinic acid-based compound, a phosphine oxide-based compound, a phosphonous acid-based compound, a phosphinous acid-based compound or a phosphine-based compound as a phosphorus compound.
However, a problem arises that, when this method is used, a certain improvement in heat resistance of the polymer is in fact seen, but when the phosphorus compound is added at an amount or more, the polymerization activity of the titanium compound is suppressed too much, a goal polymerization degree is not attained, and a polymerization reaction time is remarkably prolonged, and as a result, a color tone of the polymer is deteriorated.
JP-A No. 2000-256452 discloses a method of limiting a mole ratio (Ti/P) of the titanium compound and the phosphorus compound in a range. According to this method, inactivation of the titanium compound catalyst can be in fact prevented, but heat resistance and a color tone at a level or higher can not be obtained.
In addition, JP-A No. 2004-124067 discloses a method of separating an interval between addition of the titanium compound and addition of the phosphorus compound. However, also in this method, inactivation of the titanium compound due to the phosphorus compound progresses in a polymerization reaction system and, when an addition amount of the phosphorus compound is large, inactivation of the catalyst is still caused.
JP-A No. 7-138354 discloses improvement in melt thermal stability and a color tone of the resulting polyester by coordination of the titanium compound and the phosphorus compound, but the improving effect is not sufficient.    Patent Literature 1: JP-A No. 2004-292657 (claims)    Patent Literature 2: JP-A No. 2000-256452 (claims)    Patent Literature 3: JP-A No. 2004-124067 (claims)    Patent Literature 4: JP-A No. 7-138354 (claims)